The present invention relates to a fuel injection system for self-ignition internal combustion engines, in which a fuel volume to be injected is divided into a pre-injection volume and a main injection volume. The fuel injection system comprises an line injection pump and an injection valve. Fuel is supplied from the line injection pump to the injection valve via two injection lines of different lengths, whereby a shorter one of the injection lines, before connecting with the injection valve, is provided with a spring-loaded first check valve.
An effective means for reducing the combustion noise of Diesel engines is the so-called pre-injection. When for this purpose injection devices with conventional fuel injection pumps, which are driven by the cam shaft of the engine, are used (concept of the displacement piston), the following problems are encountered: the respective connecting line between the pumping element and the valve which are relatively long display, besides a pressure dependent volume storage capacity (as a result of the volume compressibility of the fuel) also the usual transfer behavior, due to wave mechanics as a response to the fast volume injection of the fuel. This may be partially solved by adequately dimensioning the stroke of the pumping element piston, but, due to the terminal pressure propagation in the injection line at the speed of sound the detrimental results of the time delay in making available the injection volume may not be overcome.
In order to divide the injection process into a pre-injection phase and a main injection phase it is known from U.S. Pat. No. 4,711,209 to have two injection lines of different lengths which originate at one line injection pump. A first injection line leads directly to a dosage valve unit having a cylinder and a piston, while a second injection line is branched off directly before the dosage valve unit and is connected, via a check valve, to a line originating at the dosage valve unit and opening into the injection valve. Due to the longer first injection line the piston of the dosage valve unit is moved when the pumping action begins and a volume of fuel which corresponds to the cylinder volume is pre-injected. Due to the extension of the first injection line by the second injection line, the main injection is delayed by the additional time required to pass the lengths of the lines. In order to avoid backflow a check valve is provided in the second injection line. A disadvantage of such an arrangement as described above is that there is no flexible, external control of the volume and the starting point of the pre-injection and the main injection Also, there is the volume storage capacity of the injection line to be dealt with, because the fuel may not be considered incompressible any longer.
Injection devices which realize such a control, at least for the main injection, with the aid of two electromagnetically actuated by-pass valves are known from a brochure by Klockner-Humboldt-Deutz (KHD) published in 1985. With one of these valves, installed in the vicinity of a pumping element, the pressure increase in the injection line is started upon initiation of the valve closing phase (which causes fuel to be injected into the combustion chamber), while a second valve, in the vicinity of the valve holder, serves to initiate a pressure collapse at the injection valve by opening a by-pass so that the injection process is interrupted. An electronic control unit serves to accordingly control the injection (one per working cycle) from the beginning to the end. When the by-pass is closed by the by-pass valve which is situated in the vicinity of the pumping element, the pressure increase phase in the injection line is started. The resulting pressure wave, after the time required to travel the length of the injection line at the speed of sound, leads to the lifting of the valve needle and the subsequent injection process. The injection results in the reduction of the fuel volume at the end of the injection line which is facing the valve holder. Especially at low engine revolutions this may lead to a pressure collapse of an order of magnitude that causes a temporary closing of the valve.
Therefore, in the case of a desired pre-injection this fuel volume reduction should be compensated for by a buffer fuel volume, which is in immediate supply and replenishable, in the vicinity of the valve holder during any operational stage of the engine. This buffer volume of fuel must at least of the same amount as the sum of the fuel volumes of the pre-injection and the by-pass volume (which occurs between the pre-injection and the start of the main injection). To provide a fuel surplus for the pre-injection (by selecting a respective height and a respective slope of the cam of the injection pump) is useful then when the initiation of the termination of the pre-injection is forced in a known manner by a second electromagnetically controlled by-pass valve which is provided in the close vicinity of the valve holder.
The aforementioned pressure collapse independent of its initiation by controlled or uncontrolled pre-injection, is to be expected every time the outlet volume stream is greater than the feed volume stream. This is usually the case with slow displacement speeds of the piston (as a component of the internal injection element of the pump), i.e., when the engine is running at low revolutions and when the lifting stroke of the cam is small. This pressure collapse is especially disadvantageous when, according to practical requirements, a short time interval is desired between the pre-injection and the main injection. The recuperation of the pressure at the injection valve, which also is a factor determining the starting of the main injection, mostly depends on the pressure development up-stream, which is timed according to the speed of sound and is not influenced by the volume removal due to the pre-injection, and may result in a relatively long time span for the pressure build-up at the valve, thereby causing an intolerable great delay of the main injection.
It is therefore an object of the present invention to compensate for the negative influences of the compressibility of the fuel on the injection process which is defined as a function of time and further to suppress the disturbing wave mechanical effects.